Various types of pumps are utilized in fluid transporting systems in order to develop and maintain a desired amount of flow energy in the fluid. Many of these pumps require for their operation at least one rotatable shaft to drive a mechanical energy-transferring device such as a piston, impeller, or gear. Typically, the rotational power or torque transmitted to the shaft is generated in a motor disposed in remote relation to the pump housing. One example is a gear pump, which is utilized in a number of well known applications to meter and discharge various types of fluids.
The gear pump may generally be described as being a rotary, positive displacement pump. In its most basic design, the gear pump includes a pair of intermeshing spur, single-helical or double-helical (i.e., herringbone) gears disposed in a housing having tight internal dimensional tolerances. One gear serves as the driving gear and is rotatable with a drive shaft (i.e., the shaft powered by a motor). The other gear serves as the driven gear and is rotatable on an idler shaft. The shafts are usually mounted in journal bearings on each side of the gears. In operation, the gears create a pressure differential between a suction side and a discharge side of the gear pump housing. The working fluid is drawn into the housing at the suction side, is carried by the teeth of each gear in spaces defined by the teeth and one or more internal surfaces of the housing, and is squeezed out on the discharge side. This design results in a relatively constant rate of fluid flow with a minimum of drifting or slippage. The flow rate is dependent on gear rotational speed, but is largely unaffected by viscosity variations and pressure differential variations across the gear pump.
The performance characteristics of the gear pump make it especially useful in the processing of high-shear polymers such as rubber, PVC, and EDPM, where pressure, volume and uniformity of the flowing material must be controlled. For example, the gear pump may be used to transport synthesis polymeric material from a reaction vessel. The gear pump may also be used in connection with an extruder. A typical extruder includes an elongate barrel containing a rotating auger or screw. A hopper feeds pellets or granules of the polymeric material to the barrel, where the material is heated and melted as it is forced along the length of the barrel by the screw. In such an application, the gear pump is installed between the extruder and an extrusion die to pressurize and meter the polymer melt flow, and to dampen any pressure fluctuations or surges caused by the rotating screw of the extruder. Because the gear pump moves fluid more efficiently than the extruder and reduces the load on the extruder, the gear pump itself can be used to develop the high pressure needed in the fluid line. This enables the discharge pressure of the extruder to be separately adjusted to a reduced level in better accord with the extruder's own optimal operating point. Finally, the gear pump may be installed in line with two or more extruders as part of a compounding or mixing process to obtain similar advantages.
In polymeric material processing, the bearings selected for the gear pump are typically hydrodynamic and preferably self-lubricating. That is, instead of using a separate lubrication method such as a forced oil circulation system, the gear pump and bearings are designed with flow paths for diverting a portion of the incoming polymer melt flow and circulating that portion between the bearings and shafts prior to discharge from the gear pump. The diversionary lubricant flow path may originate on the low-pressure suction side of the pump or on the high-pressure discharge side. In either case, as the shaft rotates and polymeric material is forced into the flow path, the diametrical clearance existing between the journal area of the bearing and the outer surface of the shaft permits a wedge-shaped polymeric film to develop therein. As a result, a hydrodynamic pressure is generated in the film that is sufficient to float the journal portions of the shafts and support the loads applied to them.
Because the film is wedge-shaped, the journal portion of each shaft rotates eccentrically rather than concentrically with respect to the bearing, the eccentricity being defined as the distance between the cross-sectional center of the shaft and the center of the bearing. A minimum film thickness will occur substantially coincident with a line running through the centers of the shaft and bearing. The performance of the bearing during operation of the gear pump will depend on this minimum film thickness, as well as on the viscosity of the lubricating polymer, the adhesion of the polymer to the surfaces of the journal and the bearing, the load on the bearing surfaces, the rotational speed of the shaft, the dimensions of the bearing, the applicable coefficients of friction, the flow rate of the polymer through paths designed between the bearing and journal, and the temperature rise of the lubricant.
It is well-known that frictional heat energy is produced as the journal portion of the rotating shaft does mechanical work on the polymeric film and induces shear stresses therein. Accordingly, the shear-sensitive polymeric material may become degraded and the film strength compromised, thereby reducing the efficacy of the polymeric material as a lubricating medium. Prior designs of self-lubricating bearings have not adequately addressed this problem. Examples of such prior designs are described below. It will be appreciated, then, that improvements in self-lubricating journal bearings are continuously being sought in order to induce less shear in the lubricating material, provide more effective lubrication, improve flow to the journal portion of the bearing, and provide more efficient output rates for the gear pump.
The present invention is therefore provided to solve these and other problems associated with the effective lubrication in the journal bearings of rotating shafts in general, and specifically with the high-pressure lubrication in hydrodynamic, self-lubricating journal bearings of rotating shafts utilized in gear pumps operating in polymer processing applications under high-pressure and high-temperature conditions.